The invention relates to an MR method for suppressing MR signals from peripheral regions situated outside an isocenter.
The invention also relates to an MR apparatus for carrying out such a method and to a coil system which is suitable for that purpose.
MR images which are to reproduce a given field of view (FOV) are liable to contain artefacts which originate from undesirable MR signals which occur in peripheral regions in which the conditions for magnetic resonance are satisfied as well as in the FOV. Particularly in MR apparatus comprising comparatively short gradient coils it may occur that the gradient of the magnetic field generated by these coils decreases and reverses its sign already at a small distance from the isocenter. In the peripheral regions in which such reversal occurs the so-called B0 field may still be so strong that the magnetic resonance conditions are also satisfied therein, giving rise to undesirable MR signals when these peripheral regions are covered by the RF magnetic field. It is not possible to saturate the nuclear magnetization in the peripheral regions in conformity with DE-A 36 04 281 (PHD 86-018), because for the stated variation in space of the gradient the magnetic field in the peripheral regions may be of the same magnitude as in the region around the isocenter.
Therefore, it is an object of the present invention to provide a method in which the MR signals from these peripheral regions can be effectively suppressed. This object is achieved according to the invention by taking the following steps:
a) generating a location-dependent, brief steady magnetic field whose field strength in the peripheral regions is either larger or smaller than its field strength at the isocenter,
b) generating at least one RF pulse during the application of the magnetic field, which RF pulse excites the nuclear magnetization in the peripheral regions but not in the isocenter,
c) dephasing the excited nuclear magnetization.
The invention is based on the consideration that when a location-dependent, brief steady magnetic field can be generated whose field strength in the peripheral regions is either larger or smaller than that at the isocenter, it will always be possible to generate an RF pulse which excites the nuclear magnetization only in the peripheral regions but not at the isocenter. Subsequently, the excited nuclear spins in the peripheral regions can be dephased so that (for the time being) MR signals can no longer occur therein due to this saturation.
The term xe2x80x9cbrief steady magnetic fieldxe2x80x9d is to be understood to mean hereinafter a magnetic field which does not change its polarity during a period of time amounting to at least a few periods of the RF oscillation contained in the RF pulse. The term xe2x80x9cbriefxe2x80x9d will also be omitted hereinafter when no mix-ups with the steady (B0) main magnetic field are to be expected. Typically, the brief steady magnetic field is constant during a period of time of the order of magnitude of 1 ms.
The embodiment disclosed in claim 2 ensures that the MR examination subsequent to the saturation is not influenced by the location-dependent brief steady magnetic field.
Claim 3 discloses an MR apparatus for carrying out the method disclosed in claim 1, including a main field magnet for generating a uniform, steady magnetic field. The location-dependent, steady magnetic field is then generated by means of an (additional) saturation coil system which is constructed in such a manner that the magnetic field in the peripheral regions to both sides of the center of this coil is either stronger or weaker than that at its isocenter. Such a saturation coil system may have the same length as, or may even be shorter than, the gradient coil whose field variation is liable to give rise to the undesirable MR signals claim 4 describes a preferred embodiment of such an MR apparatus. When the two lateral coil sections have the same number of turns and are situated at the same distance from the central coil section, a symmetrical field variation will be obtained. However, it is alternatively possible to realize a non-symmetrical variation by changing of the distance or the number of turns.
The gradient coil system in the embodiment described in claim 5 is interleaved in space with the saturation coil system so that no additional volume is required for the saturation coil.
Claim 6 describes a coil system which is suitable for use in an MR apparatus for carrying out the method according to the invention.